The present invention relates generally to a zig zag sewing machine, and more particularly to a sewing machine having an arrangement for tracing the edge of a workpiece (hereinafter referred to as "fabric edge") and sewing a line of stitches a constant predetermined distance away from the fabric edge.
A zig zag sewing machine having an arrangement for following the fabric edge to sew a seam along the fabric edge is known according to the disclosure in U.S. Pat. No. 4,248,168. In the disclosed sewing machine, the location of the fabric edge is sensed by an edge sensor, and the lateral needle jogging mechanism is controlled in response to a signal generated by the sensor, so as to sew a line of stitches a predetermined distance from the fabric edge.
As described in the above-identified U.S. Patent, a zig zag sewing machine which inherently has a lateral needle jogging function to change the needle position in the direction normal to the feed direction of a work fabric, may be adapted to sew a line of stitches along the edge of the fabric while following or tracing the fabric edge, by adding a sensor to detect the fabric edge, and other simple devices responsive to the sensor.
In the arrangement disclosed in the above-identified U.S. Patent, the edge sensor is of a light-transmission type including a photosensor which is mounted on a work bed and whose output signal corresponds to an area which is covered by the work fabric. The needle is moved in the lateral direction by a distance corresponding to the level of the output signal. For accurate sensing of the fabric edge, therefore, the edge sensor must have a linearity of the output signal at least over an area of detection within which the fabric edge is expected to be moved. To this end, the sewing machine must use an expensive sensor for accurate detection of the fabric edge. Further, the control device responsive to the edge sensor must have hardware or software provisions for converting the output signal of the photosensor into a lateral jogging motion of the needle. In this respect, the control device is complicated and accordingly costly.
Another inconvenience associated with the light-transmission type of edge sensor is derived from the fact that a given amount of light transmitted through the work fabric is inevitably received by the photosensor. The amount of incidence of this undesirable light upon the photosensor is remarkably influenced by the thickness, color, material and other parameters of the work fabric. To avoid the influence or compensate for a variation in the sensor output due to the varying parameters of the work fabric, a secondary photosensor must be disposed so as to be always covered by the work fabric. The use of the secondary sensor increases the cost of the sewing machine. Furthermore, where the work fabric is a checkered or otherwise figured cloth, the edge sensing primary photosensor and the compensating secondary photosensor may be located below different color portions of the figured cloth. In this case, the secondary photosensor fails to achieve a proper amount of compensation, causing an error in the distance between the fabric edge and the line of stitches to be formed.